The invention relates to an electrooptical liquid crystal system
which between 2 electrode layers contains a PDLC film comprising a liquid crystal mixture being dispersed in form of microdroplets in an optically isotropic, transparent polymer matrix, PA1 in which one of the refractive indices of the liquid crystal mixture is matched to the refractive index of the polymer matrix, and PA1 which in one of the two switching states has a reduced transmission compared with the other state independent of the polarization of the incident light. PA1 a high HR, PA1 a Swiss cheese morphology and PA1 an excellent matching of indices resp. a high contrast and/or, in particular, a high on-state clarity. PA1 a) about 30-85 wt. % of a liquid crystal mixture containing one or more compounds of the formula I ##STR2## in which Z.sup.1 and Z.sup.2 independently of one another, are a single bond, --CH.sub.2 CH.sub.2 --, --COO--,--OCO-- or --C.tbd.C--, ##STR3## independently of one another, are each trans-1,4-cyclohexylene, 1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene or 3,5-difluoro-1,4-phenylene and one of ##STR4## can also be pyrimidine-2,5-diyl, pyridine-2,5-diyl or trans-1,3-dioxane-2,5-diyl, PA1 X.sup.1 and X.sup.2 independently of one another, are each H or F, PA1 Q is CF.sub.2, OCF.sub.2, C.sub.2 F.sub.4, OC.sub.2 F.sub.4 or a single bond, PA1 Y is H, F, Cl or CN, PA1 n is 0, 1 or 2 and PA1 R is alkyl having up to 13 C atoms, in which one or two non-adjacent CH.sub.2 groups can also be replaced by --O-- and/or --CH.dbd.CH--, PA1 b) about 15-68 wt. % of a precursor of a polymer matrix, the precursor comprising at least PA1 c) about 0.01-5 wt. % of a radical photoinitiator PA1 which between 2 electrode layers contains a PDLC film comprising a liquid crystal mixture forming microdroplets in an optically isotropic, transparent polymer matrix, PA1 in which one of the refractive indices of the liquid crystal mixture is matched to the refractive index of the polymer matrix, PA1 which in one of the two switching states has reduced transmission compared with the other state independent of the polarization of the incident light, PA1 whose precursor of the PDLC film is selected as stated above. PA1 a) 35-85 wt. % of a liquid crystal mixture containing at least one compound of formula I PA1 b) 15-68 wt. % of the precursor of the polymer matrix at least comprising PA1 c) 0.01-5 wt. % of a radical photoinitiator with the mass ratios given under a), b) and c) being related to the mass of the precursor of the PDLC film and the mass ratios of the components A, B and C relating to the mass of the precursor of the polymer matrix. PA1 Z is independently from each other a single bond or --CH.sub.2 CH.sub.2 --, PA1 l and m are independently from each other 0 or 1, and ##STR13## denotes 1,4-phenylene, 2-fluoro-1,4-phenylene or 3-fluoro-1,4-phenylene. PA1 51.0% of 4-pentyl-4'-cyanobiphenyl PA1 25.0% of 4-heptyl-4'-cyanobiphenyl PA1 16.0% of 4-octoxy-4'-cyanobiphenyl PA1 8.0% of 4-peptyl-4'-cyanoterphenyl PA1 that the liquid crystal mixture is partly dissolved in the cured polymer, and PA1 that the precursor is not completely reacted with a part of it remaining in the liquid crystal droplets PA1 LCI liquid crystal mixture I (see composition below) PA1 EHA ethyl hexyl acrylate PA1 E 270 Ebecryl 270 (commercially available polyurethanediacrylate oligomer) PA1 TMPTA 2-ethyl-2-(hydroxymethyl)-1,3-propandiol triacrylate PA1 TMPTMP trimethylolpropane tri(3-mercaptopropionate) PA1 D1173 Darocur.RTM. 1173 (a radical photoinitiator available through E. Merck, Germany) PA1 a component A containing at least 5% of one or more at least difunctional thiol monomers and/or oligomers, PA1 a component B containing at least 10% of one or more at least difunctional monomers and/or oligomers of the ene type, PA1 a component C containing at least 3 wt. % of one or monofunctional monomers and/or oligomers of the ene type with a molar mass of less than 250 g/mol, and PA1 optionally a component D containing one or more polymerizable compounds other than ene-type or thiol-type compounds, with the percentages given being related to the mass of the precursor of the polymer matrix.
In PDLC films (polymer dispersed liquid crystals) which are described, for example, in U.S. Pat. No. 4,688,900, Mol. Cryst. Liq. Cryst. Nonlin. Optic, 157, 1988, 427-441, WO 89/06264 and EP 0,272,585, one of the refractive indices of the liquid crystal mixture, customarily the ordinary refractive index n.sub.o, is selected in such a way that it more or less coincides with the refractive index n.sub.p of the polymeric matrix. If no voltage is applied to the electrodes, the liquid crystal molecules in the droplets exhibit a distorted alignment, and incident light is scattered at the phase boundary between the polymeric and liquid crystal phases.
On applying a voltage, the liquid crystal molecules are aligned parallel to the field and perpendicular to the E vector of the transmitted light. Normally incident light (viewing angle .theta.=0.degree.) now sees an optically isotropic medium and appears transparent.
No polarizers are required for operating PDLC systems, as a result of which these systems have high transmission. PDLC systems provided with active matrix addressing have been proposed on the basis of these favorable transmission properties in particular for projection applications, but in addition also for displays having high information content and for further applications.
A series of matrix materials and polymerization processes have hitherto been proposed for producing PDLC systems. In the so called PIPS technology (=polymerization-induced phase separation) the liquid crystal mixture is first homogenously mixed with monomers and/or oligomers of the matrix-forming material; phase-separation is then induced by polymerization. Differentiation must further be made between TIPS (temperature-induced phase separation) and SIPS (solvent-induced phase separation) (Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 (1988) 427) both being also methods to produce PDLC films.
The process of preparation must be controlled very carefully in order to obtain systems with good electrooptical properties. F. G. Yamagishi et al., SPIE Vol. 1080, Liquid Crystal Chemistry, Physics and Applications, 1989, p.24 differentiate between a "Swiss cheese" and a "polymer ball" morphology. In the latter one, the polymer matrix consists of small polymer particles or "balls" being connected or merging into each other while in the Swiss cheese system, the polymer matrix is continuous and exhibits well defined, more or less spherical voids containing the liquid crystal. The Swiss cheese morphology is preferred because it exhibits a reversible electrooptical characteristic line while the polymer ball system shows a distinct hysteresis generally leading to a drastic deterioration of the electrooptical characteristic line when comparing the virgin and the second run.
According to Yamagishi et al., loc. cit., the Swiss cheese morphology is promoted in cases where the polymerization reaction runs via a step mechanism, and in WO 89/06264 it is pointed out that the step mechanism is favored in cases where the precursor of the polymer matrix consists of multifunctional acrylates and multifunctional mercaptanes.
When using--as was suggested in WO 89/06,264--a precursor of the polymer matrix containing multifunctional thiols and multifunctional ene compounds, i.e., multifunctional acrylates, it was observed by the present inventors that matching of the refractive indices is often difficult. This is because the thiols have a relatively high refractive index due to the high polarizibility of the sulphur atom. Trimethylolpropane tri(3-mercaptopropionate), for example, has a refractive index of 1.52, whereas multifunctional enes usually exhibit a lower refractive index typically of 1.47-1.51.
When curing a precursor of a polymer containing a multifunctional thiol and a multifunctional ene compound in substantially stoichiometric amounts, the resultant polymer usually exhibits an index of refraction which is lower than that of its pure thiol component but still rather high and typically about 1.50 or more. It is true that reducing the ratio of the thiol component gives polymer materials with a lower index of refraction but, on the other hand, in the case of PDLC films, the ratio of the thiol component must not be chosen too low if reliable formation of a Swiss cheese morphology is to be obtained. After curing, the commercially available polymer precursor NOA 65 (manufactured by Norland Products) which is widely used for the preparation of PDLC systems, exhibits a refractive index of 1.525 which is higher than the ordinary index of refraction of most liquid crystals which typically amounts to 1.49-1.52.
In PDLC systems, another complication is that the liquid crystal mixture usually tends to dissolve into the polymer matrix to a lesser or higher degree. In the polymer matrix, the liquid crystal acts as an isotropic material exhibiting a medium refractive index given via EQU n.sup.2 =1/3(n.sub.e.sup.2 +2n.sub.o.sup.2).
Inserting typical indices of refraction of a liquid crystal mixture of n.sub.o =1.52 and n.sub.e =1.75 yields n=1.6. This phenomenon therefore leads to a further increase of the refractive index of the matrix, and the present inventors found quite generally that in order to obtain good transmission in the PDLC film, the refractive index of the precursor of the polymer matrix should be somewhat or even substantially lower than the ordinary index of refraction of the liquid crystal mixture.
With thiol-ene based precursor systems known hitherto matching of the refractive indices often is not possible or only possible to an unsatisfactory degree. A distinct improvement of matching by a corresponding design of the liquid crystal mixture generally is not possible because typical liquid crystal compounds exhibit an ordinary index of refraction considerably lower than that of thiols. The term "matching of refractive indices" does not necessarily mean that the refractive index of the polymer matrix and the ordinary index of refraction of the liquid crystal mixture are equal but is to be understood that a high maximum transmission and a low minimum transmission, i.e. a high contrast, and, in particular, a high maximum transmission or clarity of not less than 0.80 and especially of more than 0.82 are to be obtained.
The liquid crystal mixture used in PDLC films preferably has a positive dielectric anisotropy but the use of dielectrically negative liquid crystal mixtures (see, for example, WO 91/01511) or two-frequency liquid crystal mixtures (see, for example, N. A. Vaz et al., J. Appl. Phys. 65, 1989, 5043) is also discussed.
Furthermore, the liquid crystal mixture should have a high clearing point, a broad nematic range, no smectic phases down to low temperatures and a high stability and should be distinguished by an optical anisotropy .DELTA.n and a flow viscosity .eta. which can be optimized with respect to the particular application, and by a high electrical anisotropy.
Electrooptical systems containing PDLC films can be addressed passively or actively. Active driving schemes employing an active matrix having nonlinear addressing elements integrated with the image point, are especially useful for displays with high information contents. The nonlinear elements used for preparing the active matrix type electrode film can have 2 (for example, MIM or MSI diodes, metal-insulator-metal or metal-siliconnitride-indium tin oxide) or 3 (for example, TFT, thin film transistors) connections.
More details on the addressing of liquid crystal displays by an active matrix of nonlinear elements can be found, for example, in A. H. Firester, SID, 1987, Society for information Display Seminar, Seminar 5: Active Matrices for Liquid Crystals, E. Kaneko, Liquid Crystal Displays, KTK Scientific Publishers, Tokyo, Japan, 1987, chapter 6 and 7 or P. M. Knoll, Displays, Heidelberg, 1986, p. 216 ff.
When the PDLC system is addressed by means of an active matrix, a further far reaching criterion is added to the requirements listed so far which must be fulfilled by the cured polymer and the liquid crystal mixture being embedded in microdroplets. This is related to the fact that each image point represents a capacitive load with respect to the particular active nonlinear element, which is charged at the rhythm of the addressing cycle. In this cycle, it is of paramount importance that the voltage applied to an addressed image point drops only slightly until the image point is again charged in the next addressing cycle. A quantitative measure of the drop in voltage applied to an image point is the so-called holding ratio (HR) which is defined as the ratio of the drop in voltage across an image point in the nonaddressed state and the voltage applied; a process for determining the HR is given, for example, in Rieger, B. et al., Conference Proceeding der Freiburger Arbeitstagung Flussigkristalle (Freiburg Symposium on Liquid Crystals), Freiburg 1989. Electrooptical systems having a low or relatively low HR show insufficient contrast.
It is true that considerable efforts have already been undertaken hitherto to optimize PDLC systems with respect to the precursor of the polymer matrix and the liquid crystal mixture used. On the other hand, however, it is still an open problem how to reliably obtain PDLC films which are characterized both by a Swiss cheese morphology and an excellent matching of refractive indices, i.e., a high contrast and/or, in particular, a high on-state clarity.
Furthermore, only few investigations of PDLC systems having active matrix addressing can be found in the literature, and no concepts have so far been proposed for providing electrooptical systems having
Consequently, there is a high demand for non-actively addressed PDLC systems Which fulfill to a large extent the requirements described and which exhibit both a Swiss cheese morphology and an excellent matching of refractive indices resp. a high contrast and/or, in particular, a high on-state clarity. Furthermore, there is a high demand for actively addressed PDLC systems which exhibit a high HR in addition to these properties.